Speaking from my own personal experience...
My first upright cab had a 19 inch horizontal monitor in it.
Horizontal games looked mint and the vertical games were good enough to play, but after playing a quick game of Galaga on a 19inch horizontal montior one night while I was out... I NEEDED to have the games El Mondo...
Don't get me wrong, the game's scale will be proportional (just with the letterboxes on the sides.) If you're running a powerhouse of a computer you might consider enabliong a bezel to fill up the black space. There are some tweaked bezels out there that are sized to fit on the sides of verical games. (Enabling bezels will tax your system

)
I noticed that you mentioned that "rotation was not an option" so I assume that you don't have a choice at this point.
From what I've caught on the board, people running 27 inch monitors have mentioned that the vertical games look the size of a 19 inch vertical (or close to it.)
I guess you could do a 27 inch horizontal monitor if not in this project but in a future one.
For me, the cocktail idea worked pretty mint, I can play just about anything and have the best of both worlds. (Plus 4 way games of gauntlet, paddlemania, super sprint, etc.) I pretty much "sit and rotate"

If you build another cab, you could go vertical and add 2 rotary sticks in leiu of standard 8 ways to open up games like Ikari Warriors and stuff.
There's my ramble for the night.

Smack